This invention relates generally to control devices as may be used in electrical switches or fluid valves or the like and in particular to thermal actuators, methods of driving such, and actuating systems in which such thermal actuators may be employed.
In the past, various thermal actuators and actuating systems in which such thermal actuators are incorporated have been provided. Most of the past thermal actuators, as illustrated for instance in U.S. Pat. No. 2,427,900, had a container or boiler with a vaporizable liquid therein adapted to be heated by various means well known in the art to vaporize the vaporizable liquid, and the boiler was communicated through a relatively small opening with an expansible chamber also filled with the vaporizable liquid and having a wall free to move in response to changing pressures. When the boiler and the liqid therein were heated in response to the occurrence of a certain condition to effect the vaporization of the liquid, the established vapor increased the pressure in the boiler forcing the liquid therefrom through the opening into the expansible chamber moving its movable wall for transmitting an actuating or output force to operated various electrical switch or fluid valve components. Of course, the boiler and expansible chamber were generally thermally insulated from each other thereby to effect cooling of the transferred liquid from the heated boiler to the relatively cooler environs of the expansible chamber, and upon termination of the heating of the boiler in response to the occurrence of another certain condition, the transferred liquid returned thereto effecting contraction or collapse of the expansible chamber thereby to eliminate the output force transmitted by the movable wall of the expansible chamber. One of the disadvantageous or undesirable features of the past thermal actuators was that it was necessary to heat the entire thermal mass of both the boiler and liquid therein to effect the vaporization of the liuqid for actuating the thermal actuating means. This, of course, resulted in the ancillary disadvantageous or undesirable feature of necessarily increasing the size and/or output of the means utilized to heat the thermal mass in order to attain operations of the thermal actuator at faster rates of response or speeds.